I read this post last week and happen to bring my 08 LS460 in for it's 100 mile check, I mentioned this to the service manager about Lexus recommending "Synthetic oil" and even ships them from factory with Synthetic in the car. He imediately told Not True, comes through with 5w30 and all oil changes are done with 5w30, unless the owner requests and pays the difference for it!
To verfiy I contacted Lexus Cust service who said the exact same thing, these vehilcles do not come from the factory with anything other than 5w30 in the oil pan, main reason $$$ too expensive, I would take whatever that manger tells in future with a grain of salt.
I have used Synthetic oils in the past and favor Castrol Sentec over Mobil 1. :o